1. Field of the Invention
This invention relates to the transmission of digital data, and in particular to method and apparatus for reduction of distortion incurred by representations of digital data during passage through transmission links.
2. Description of the Related Art
It is well known in the field of transmission of digital data, and particularly for transmission of data concerning digital broadcast material, for modulation techniques to use symbols, arranged as points in a particular constellation pattern to represent digital data. Typical techniques are those of phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). Two of the more common schemes of these techniques are those of Quadrature PSK (QPSK), e.g. digital satellite transmission for “direct to home” applications, and 8PSK for e.g. satellite news gathering applications. There is a recognized desire to utilize higher order modulation schemes, such as 16 PSK and 16 QAM. This will allow transmission at a higher bit rate, thus providing the opportunity to carry a greater number of channels within a pre-defined bandwidth of a particular transmission link.
Transmission of a modulated signal through transmission links such as terrestrial, satellite or cable links result in distortion of the signal. This distortion is due, in part at least, to the non-linear effects of passage of a signal through the transmission link. Distortion leads to a change in location of the constellation points of any given modulation scheme. An increase in the order of modulation results in a decrease in the distance between constellation points, and so leads to a higher probability of distortion leading to errors occurring during the demodulation of higher order modulation schemes.
Prior art methods used to reduce the effects of distortion by non-linear component(s) within transmission links include use of at least partially compensating pre-correction. One approach is that of feed-forward, where the non-linear output of an amplifier is sampled, and compared with the required signal prior to transmission, with the resultant error being subtracted from the amplifier output. This approach is only suitable for systems in which the pre-distorter and amplifier are co-located. Thus, where this technique is used for satellite transmission links the opportunity for introduction or modification of the pre-distorter to take account of changes in amplifier characteristics is severely limited.
Another technique is that of constellation pre-distortion, where the constellation points generated by the modulator are pre-distorted such that at the amplifier output the constellation points are located in their correct relative positions. This method is suitable only for transmission links that are memory-less. This precludes the use of this approach where pulse shaping takes place before non-linear amplification of the signal. Therefore, it is not suitable for those transmission links that include bandpass filtering of the signal.
Signal pre-distortion performed at the radio (RF), intermediate (IF) or base band frequencies is often carried out by application of an inverse function of the distortion to the signal as disclosed in WO-A-95132561 and U.S. Pat. No. 4,992,754. This type of pre-correction generates out of band components, which are then carried through to the amplifier input. Where the amplifier has an input filter, as is common for amplifiers used in satellite transmission links, then these components may be removed from the signal which becomes the input to the amplifier. Thus, the distortion imposed by the amplifier will not be accurately corrected as the amplifier input signal is not the entire transmitted signal. This means that this form of pre-correction is not effective for correction of amplifiers contained within satellite transponders where the bandwidth of the incoming signal is high in relation to the bandwidth of the transponder. Additionally, for digital transmission using higher order modulation schemes, this type of pre-correction requires very high clocking rates in order to generate the wide-band pre-distortion components.